Parts such as pistons, connecting rods and brake rotors, for use in the internal combustion engines of motor vehicles, motorcycles and the like must have a strength capable of withstanding intensive motion at high temperatures.
On the other hand, attempts have been made in recent years to reduce the weight of motor vehicles and the like from the viewpoint of savings of energy, and it is therefore desired to make the components thereof lightweight. Accordingly, aluminum alloys are placed into wide use for the above-mentioned parts for which a high strength is required at high temperatures of at least about 300.degree. C.
Such an Al alloy having an excellent high-temperature strength is known and is prepared by solidifying a molten aluminum alloy at a high cooling rate of less than 10.sup.5 .degree. C./sec by gas atomization to obtain an Al alloy powder and shaping the powder by hot working (Unexamined Japanese Patent Publication No. 47448/1987).
Also available is a material which is prepared by subjecting a rapidly solidified Al alloy powder containing at least 13 wt. % of Si to hot plastic working to join the particles together.
The Al alloy powder is formed with an oxide film over the surface, so that when the powder is subjected to a usual hot-working process, the oxide film is present between the joined particles. Consequently, the joint between the particles is unsatisfactory which causes the problem that the alloy as shaped is deficient in high-temperature strength.